1. Field of the Invention
The present invention is directed to the field of devices used for measuring stress build up in vapor deposition coatings and more specificially to the area of measuring such coatings utilizing a capacitive sensor.
2. Description of the Prior Art
In applying metal or dielectric coatings to work pieces by the use of sputtering systems, such as the one shown in commonly assigned U.S. Pat. No. 4,525,264 (incorporated herein by reference), it has been found that some degree of residual stresses remain in the coating. The detection and control of such coatings stresses or "film stresses" is seen as highly desirable in order to prevent resulting failures or unwanted effects in the coatings. By being able to sense the occurrance, strength and type of internal stresses developed in vapor deposited material coatings, one may thereafter control subsequent coatings by making appropriate changes in the process. For instance, by lowering the gas pressure or raising the discharge current in a sputtering system one can reverse the stresses in subsequent coatings from tensile to compressive.
One of the principle conventional ways of assessing the stress effects produced by diffeent vapor process conditions involves the generation of individual samples of coatings at each distinct setting of the various process parameters. The individual samples receiving sputtered material coatings comprise relatively thin substrates so that residual film stresses can be determined by observation. The thin substrates have a resultant curvature based opon the type and amount of stresses within the film coating. Film stresses are determined in this method by removing the coated samples from the deposition chamber for subsequent examination.
Conventionally, in-situ measurements of stress forces present in thin films prepared by sputtering techniques utilize a "cantilever" method, whereby a thin conductive substrate or plate is rigidly clamped at only one end and oriented in a position so as to receive the sputtered material. A rigidly mounted and nonmoveable reference electrode is positioned nearby the free end of the cantilevered conductive substrate so that the substrate and the electrode serve as a capacitor having a value of capacitance which varies according to the spacial distance between the two. Typically, in this prior art method, the cantilevered substrate receives a coating of material and bends either towards the rigid electrode or away from the rigid electrode depending upon the stresses present to cause such deflection. Changes in capacitance are electrically sensed and provide an indication as to the stress levels.
The cantilever measurement technique provides relative measurement from on coating to the other. However, due to the mechanical instabilities present in the set up, it is only suitable for laboratory use under ideal conditions. For instance, its use would be difficult in an industrial application where the coating quality is periodically sampled to determine stress levels of coating materials being sputtered on the surface of production parts. This is especially true where vibrations are present which would cause variations in the readings.